This invention pertains to magnetic recording and more particularly to the reproduction of non-return-to-zero magnetic recordings.
In a magnetic recording system having a high storage density such as a rotating floppy disc, the signal read out from the tracks is generally asynchronous with respect to a fixed clock because expansions, etc. of the surface due to temperature changes, mechanical effects, etc. Therefore, self clocking systems are employed. Of these self clocking systems the non-return-to-zero (NRZ) schemes are used because of their high packing densities. Typically, the methods used includes some combination of linear amplification, differentiation, zero-crossing and/or threshold detection. Variations of these combinations are shown in the following U.S. Pat. Nos. 3,164,815; 3,404,391; 3,413,625; 3,503,059; 3,719,934; 3,838,448; 3,840,892; 3,865,981; 3,909,518; and 3,947,876. (Of these patents U.S. Pat. No. 3,413,625 to Mitterer et al. is considered closest to the present invention). While many of the proposed solutions are adequate in many circumstances, they do not solve the problem known as the "shouldering effect". This occurs when the primary signal is differentiated and produces what is commonly referred to as the "droop" problem. This problem is more acute in disc technology due to the wide variation in flux densities resulting from a single frequency recording on the circular tracks with varying radial distances from the center of a disc rotating at a constant angular velocity.
More specifically the shouldering effect occurs with high resolution reproducing transducers when the time between peaks exceeds a given interval. In that case, there is a "knee" in the form of third harmonic distortion in the reproduced waveform. When this knee is differentiated it produces a droop in the differentiated signal which if it reaches a certain value causes a spurious pulse.